Sansui AU555A (Tech support plz =)

[TNR]

Hi,

I recently bought my first vintage amp (and put together my first HiFi setup).

have a Sansui AU555A which I completely love, both in terms of looks and sound. It was in all original condition so I replaced the power supply caps with Nichicon parts (slightly uprated in both voltage and capacitance). I'm pleased to say it still sounds just as nice.

However, when I tried to adjust the bias as per the service manual, I had an issue. The bias current reading keeps climbing with my meter attached to the terminals. If I set it to 20mA and keep the probes there for 2-3mins it will eventually read 40mA and keep climbing.. Then if I come back to it after some time it'll read 20mA again and start climbing up again. It's kind of like when trying to measure stuff in circuit with capacitors charging up.
I have throughly cleaned both bias adjustment VR's and both the left and right bias measurements still do this behaviour.

Oh, but I've had the system playing music for over an hour continuously and it was completely fine.

Also the DC B+ voltage is steady at 59.6V

Please help?

Anyway here is my humble setup.

 

[Binkman]

Welcome to A.k. (forum pg. 2 bump)
Please clarify the measurement method.
From a cold start does the bias settle down and stay in spec at idle with nothing connected and vol. off?

 

[TNR]

Hi, thanks for the welcome.

No speakers connected. Unit gently warm from idling for at least 40mins. Volume at a minimum. I had removed fuses F002 & F003. Measurement probes across the fuse terminals from top side of chassis.

Amp settings during measurement as follows (I also tried with speaker A switch turned on, no difference):


And measurement:



Just now I tried what you said. To re-do the measurement from a cold start up, it's exactly the same.

Here is a video showing what the meter reads. The video starts just a second or two after the unit is turned on. Note: I'm not moving the VR or even touching the amp during this measurement, it just keeps going up itself.

Thank you.

 

[Dswankey]

Welcome aboard!

There is a dedicated Sansui sub-forum here at AK, you might want to check it out as there are a lot of knowledgeable Sansui enthusiasts who should be able to assist.

https://www.audiokarma.org/forums/index.php?forums/exclusively-sansui.39/

 

[RobinTW]

I don't have 555, but its big brother 777, instead. They are similar.
A couple of suggestions.
1. Remove fuses and one by one only when you want to take measurements. If the fuses are not on, those output transistors are not powered. Therefore they are not warm up to their working temperatures.
2. Use mini clips (or something that can stay on) instead of probes.

 

[TNR]

Welcome aboard!

There is a dedicated Sansui sub-forum here at AK, you might want to check it out as there are a lot of knowledgeable Sansui enthusiasts who should be able to assist.

https://www.audiokarma.org/forums/index.php?forums/exclusively-sansui.39/

Thanks! Okay, I've just posted there, so hopefully someone knows whats going on or can point me in the right direction.

I don't have 555, but its big brother 777, instead. They are similar.
A couple of suggestions.
1. Remove fuses and one by one only when you want to take measurements. If the fuses are not on, those output transistors are not powered. Therefore they are not warm up to their working temperatures.
2. Use mini clips (or something that can stay on) instead of probes.

Hi, thank you for those suggestions. I already tried to leave the fuse in to warm it up and then remove it just before checking, but I get the same issue.
I'll try to find some clips.